This invention relates generally to semiconductor wafer processing equipment and more particularly apparatus for cooling semiconductor wafers after they have been subjected to high-temperature processing.
Integrated circuits are formed on semiconductor wafers by subjecting the wafers to a number of deposition, masking, doping and etching processes. Some of these processes occur at relatively high temperatures. For example, the deposition of of epitaxial or polysilicon layers usually requires a wafer to be heated to a temperature in the range of 600.degree. C.-1200.degree. C.
A wafer subjected to a high temperature process must often be cooled before it is further processed or stored. This is due, in part, to the requirement of many processes for a relatively cool wafer and is also due to the fact that a hot wafer is very reactive with water vapor and a variety of other gasses and therefore cannot be exposed to the ambient environment in its heated state. Also, plastic wafer storage cassettes begin to melt at about 70.degree. C., so wafers have to be cooled below that temperature before they are stored in a cassette.
A variety of semiconductor equipment manufacturers provide multi-chamber semiconductor processing systems. For example, Applied Materials, Inc. of Santa Clara, Calif. sells multi-chamber processing systems under the trademarks "Precision 5000" and "Endura 5500" which include a mainframe assembly capable of supporting four or more processing chambers. A mainframe assembly includes a gas distribution system, a vacuum system, a load-lock system, a robotic wafer handling system, and a process control computer. Each of the processing chambers is adapted to perform a specific process such as an etch, chemical vapor deposition (CVD) or a physical vapor deposition (PVD) process.
Multi-chamber processing systems are well suited for single-wafer processing, as opposed to the batch wafer processing prevalent in the past. Single wafer processing is particularly advantageous for larger wafer diameters, such as 200 mm wafers, because it is easier to obtain good process uniformity over the surface of a single wafer than it is to obtain such uniformity over the surfaces of a batch of wafers. Uniformity, both within a wafer and wafer-to-wafer, is increasingly important as the density of integrated circuitry increases.
A problem with single-wafer, multi-chamber processing systems is that the throughput of the system (i.e. the number of wafers per unit time which can be processed) is dependent upon the slowest process to which the wafers are subjected. For example, if a wafer is subjected to a high-temperature process in a particular chamber and then must remain within that chamber until the wafer is sufficiently cool to move, the throughput of the system is adversely affected.
In consequence, there is a need in the industry for a method and apparatus for cooling wafers in such a manner that it does not adversely affect the throughput of a multi-chamber processing system. The cooling method must furthermore cool the wafer in a predictable and uniform manner and within a non-reactive environment to minimize the creation of defects in the integrated circuitry being formed on the semiconductor wafer.